1. Field of the Invention
The invention relates to a coding/decoding apparatus and, more particularly, to an apparatus for variable length coding image data after it was orthogonally transformed on a block unit basis comprising a predetermined number of pixels and to a decoding apparatus for decoding the coding data which was coded by the coding apparatus.
2. Related Background Art
Generally, since an information amount of an image signal is very large, in case of recording the image signal by a digital recording and reproducing apparatus such as a digital VTR or the like, the information amount is reduced by a high efficient compression coding and the reduced image signal is recorded.
An orthogonal transformation coding using a DCT (Discrete Cosine Transform) is known as an example of the high efficient compression coding techniques.
According to the orthogonal transformation coding, an input image signal is separated into blocks and each of the frequency components obtained by frequency dividing the image signal on such a block unit basis is coded. By using a method such that a data amount which is allocated is reduced or the like with respect to the high frequency component in which an influence by the deterioration on the eyesight characteristics is small, the information amount of the input image signal can be reduced.
The case where the signal which was compression coded by the above method is recorded and reproduced by the digital VTR will now be described.
When the coded data is recorded onto a tape, it is separated into sync blocks of a predetermined data amount unit as a minimum synchronous unit in the reproducing mode and the data is recorded.
FIG. 1 shows a construction of a sync block.
In FIG. 1, SYNC 1 denotes a synchronous pattern; and ID 2 indicates identification information comprising address information or the like to indicate the position on the screen of the data recorded; and DATA 3 denotes image data which was compression coded.
FIG. 2 shows an arrangement on the tape of the sync blocks recorded by a conventional VTR.
Reference numerals 2a, 2b, and 2c in FIG. 2 denote tracks. The sync blocks are arranged in those tracks.
Each track is a locus of a recording head in the recording mode. In the normal reproducing mode, a reproducing head scans on the same locus as the locus which was scanned by the recording head in the recording mode, so that the image data in each sync block can be reproduced.
A method of reproducing the recording data in a special reproducing mode will now be described.
In FIG. 2, reference numeral 10 denotes a locus of the reproducing head in a high speed searching mode as one of special reproducing modes.
Since a tape travelling speed in the high speed searching mode is faster than that in the normal reproducing mode, the locus 10 of the reproducing head is a locus such that the reproducing head scans a plurality of tracks.
Therefore, the data of only the sync blocks shown by hatched regions in FIG. 2 can be extracted from the tape by the reproducing head.
In the above conventional recording/reproducing method of image data mentioned above, however, a sync block in which only a part can be scanned exists in the reproducing head locus 10.
For example, there is a sync block like a sync block 11 in FIG. 2 such that the whole portion cannot be scanned although only the front portion of the sync block can be scanned. Therefore, since the whole DATA arranged in the rear portion in the sync block 11 cannot be fetched, there is a problem such that the data which is necessary for the expanding process cannot be prepared and the whole image data existing in the sync block 11 cannot be reproduced.
In the case where a transmission error exceeding an error correction ability occurs on a transmission path, there is also a problem such that the data after the position at which the transmission error occurred cannot be reproduced and the data of a fixed length unit disappears in the worst case.